Lower deref patterns to MIR

This handles using deref patterns to choose the correct match arm. This
does not handle bindings or guards.

Co-authored-by: Deadbeef <ent3rm4n@gmail.com>

compiler/rustc_hir_typeck/src/mem_categorization.rs

@@ -711,13 +711,19 @@ fn cat_pattern_<F>(
                 self.cat_pattern_(place_with_id, subpat, op)?;
             }
 
-            PatKind::Box(subpat) | PatKind::Ref(subpat, _) | PatKind::Deref(subpat) => {
+            PatKind::Box(subpat) | PatKind::Ref(subpat, _) => {
                 // box p1, &p1, &mut p1. we can ignore the mutability of
                 // PatKind::Ref since that information is already contained
                 // in the type.
                 let subplace = self.cat_deref(pat, place_with_id)?;
                 self.cat_pattern_(subplace, subpat, op)?;
             }
+            PatKind::Deref(subpat) => {
+                let ty = self.pat_ty_adjusted(subpat)?;
+                // A deref pattern generates a temporary.
+                let place = self.cat_rvalue(pat.hir_id, ty);
+                self.cat_pattern_(place, subpat, op)?;
+            }
 
             PatKind::Slice(before, ref slice, after) => {
                 let Some(element_ty) = place_with_id.place.ty().builtin_index() else {

compiler/rustc_mir_build/src/build/matches/mod.rs

@@ -1163,6 +1163,7 @@ enum TestCase<'pat, 'tcx> {
     Constant { value: mir::Const<'tcx> },
     Range(&'pat PatRange<'tcx>),
     Slice { len: usize, variable_length: bool },
+    Deref { temp: Place<'tcx> },
     Or { pats: Box<[FlatPat<'pat, 'tcx>]> },
 }
 
@@ -1222,6 +1223,12 @@ enum TestKind<'tcx> {
 
     /// Test that the length of the slice is equal to `len`.
     Len { len: u64, op: BinOp },
+
+    /// Call `Deref::deref` on the value.
+    Deref {
+        /// Temporary to store the result of `deref()`.
+        temp: Place<'tcx>,
+    },
 }
 
 /// A test to perform to determine which [`Candidate`] matches a value.

compiler/rustc_mir_build/src/build/matches/test.rs

@@ -42,6 +42,8 @@ pub(super) fn test<'pat>(&mut self, match_pair: &MatchPair<'pat, 'tcx>) -> Test<
                 TestKind::Len { len: len as u64, op }
             }
 
+            TestCase::Deref { temp } => TestKind::Deref { temp },
+
             TestCase::Or { .. } => bug!("or-patterns should have already been handled"),
 
             TestCase::Irrefutable { .. } => span_bug!(
@@ -143,35 +145,11 @@ pub(super) fn perform_test(
                         );
                     }
                     let re_erased = tcx.lifetimes.re_erased;
-                    let ref_string = self.temp(Ty::new_imm_ref(tcx, re_erased, ty), test.span);
                     let ref_str_ty = Ty::new_imm_ref(tcx, re_erased, tcx.types.str_);
                     let ref_str = self.temp(ref_str_ty, test.span);
-                    let deref = tcx.require_lang_item(LangItem::Deref, None);
-                    let method = trait_method(tcx, deref, sym::deref, [ty]);
                     let eq_block = self.cfg.start_new_block();
-                    self.cfg.push_assign(
-                        block,
-                        source_info,
-                        ref_string,
-                        Rvalue::Ref(re_erased, BorrowKind::Shared, place),
-                    );
-                    self.cfg.terminate(
-                        block,
-                        source_info,
-                        TerminatorKind::Call {
-                            func: Operand::Constant(Box::new(ConstOperand {
-                                span: test.span,
-                                user_ty: None,
-                                const_: method,
-                            })),
-                            args: vec![Spanned { node: Operand::Move(ref_string), span: DUMMY_SP }],
-                            destination: ref_str,
-                            target: Some(eq_block),
-                            unwind: UnwindAction::Continue,
-                            call_source: CallSource::Misc,
-                            fn_span: source_info.span,
-                        },
-                    );
+                    // `let ref_str: &str = <String as Deref>::deref(&place);`
+                    self.call_deref(block, eq_block, place, ty, ref_str, test.span);
                     self.non_scalar_compare(
                         eq_block,
                         success_block,
@@ -270,9 +248,57 @@ pub(super) fn perform_test(
                     Operand::Move(expected),
                 );
             }
+
+            TestKind::Deref { temp } => {
+                let ty = place_ty.ty;
+                let target = target_block(TestBranch::Success);
+                self.call_deref(block, target, place, ty, temp, test.span);
+            }
         }
     }
 
+    /// Perform `let temp = <ty as Deref>::deref(&place)`.
+    pub(super) fn call_deref(
+        &mut self,
+        block: BasicBlock,
+        target_block: BasicBlock,
+        place: Place<'tcx>,
+        ty: Ty<'tcx>,
+        temp: Place<'tcx>,
+        span: Span,
+    ) {
+        let source_info = self.source_info(span);
+        let re_erased = self.tcx.lifetimes.re_erased;
+        let deref = self.tcx.require_lang_item(LangItem::Deref, None);
+        let method = trait_method(self.tcx, deref, sym::deref, [ty]);
+        let ref_src = self.temp(Ty::new_imm_ref(self.tcx, re_erased, ty), span);
+        // `let ref_src = &src_place;`
+        self.cfg.push_assign(
+            block,
+            source_info,
+            ref_src,
+            Rvalue::Ref(re_erased, BorrowKind::Shared, place),
+        );
+        // `let temp = <Ty as Deref>::deref(ref_src);`
+        self.cfg.terminate(
+            block,
+            source_info,
+            TerminatorKind::Call {
+                func: Operand::Constant(Box::new(ConstOperand {
+                    span,
+                    user_ty: None,
+                    const_: method,
+                })),
+                args: vec![Spanned { node: Operand::Move(ref_src), span }],
+                destination: temp,
+                target: Some(target_block),
+                unwind: UnwindAction::Continue,
+                call_source: CallSource::Misc,
+                fn_span: source_info.span,
+            },
+        );
+    }
+
     /// Compare using the provided built-in comparison operator
     fn compare(
         &mut self,
@@ -660,13 +686,21 @@ pub(super) fn sort_candidate(
                 }
             }
 
+            (TestKind::Deref { temp: test_temp }, TestCase::Deref { temp })
+                if test_temp == temp =>
+            {
+                fully_matched = true;
+                Some(TestBranch::Success)
+            }
+
             (
                 TestKind::Switch { .. }
                 | TestKind::SwitchInt { .. }
                 | TestKind::If
                 | TestKind::Len { .. }
                 | TestKind::Range { .. }
-                | TestKind::Eq { .. },
+                | TestKind::Eq { .. }
+                | TestKind::Deref { .. },
                 _,
             ) => {
                 fully_matched = false;

compiler/rustc_mir_build/src/build/matches/util.rs

@@ -5,8 +5,8 @@
 use rustc_infer::infer::type_variable::TypeVariableOrigin;
 use rustc_middle::mir::*;
 use rustc_middle::thir::{self, *};
-use rustc_middle::ty;
 use rustc_middle::ty::TypeVisitableExt;
+use rustc_middle::ty::{self, Ty};
 
 impl<'a, 'tcx> Builder<'a, 'tcx> {
     pub(crate) fn field_match_pairs<'pat>(
@@ -249,10 +249,15 @@ pub(in crate::build) fn new(
                 default_irrefutable()
             }
 
-            PatKind::DerefPattern { .. } => {
-                // FIXME(deref_patterns)
-                // Treat it like a wildcard for now.
-                default_irrefutable()
+            PatKind::DerefPattern { ref subpattern } => {
+                // Create a new temporary for each deref pattern.
+                // FIXME(deref_patterns): dedup temporaries to avoid multiple `deref()` calls?
+                let temp = cx.temp(
+                    Ty::new_imm_ref(cx.tcx, cx.tcx.lifetimes.re_erased, subpattern.ty),
+                    pattern.span,
+                );
+                subpairs.push(MatchPair::new(PlaceBuilder::from(temp).deref(), subpattern, cx));
+                TestCase::Deref { temp }
             }
         };
 

tests/ui/pattern/deref-patterns/bindings.rs

@@ -0,0 +1,37 @@
+//@ run-pass
+#![feature(deref_patterns)]
+#![allow(incomplete_features)]
+
+fn simple_vec(vec: Vec<u32>) -> u32 {
+    match vec {
+        deref!([]) => 100,
+        // FIXME(deref_patterns): fake borrows break guards
+        // deref!([x]) if x == 4 => x + 4,
+        deref!([x]) => x,
+        deref!([1, x]) => x + 200,
+        deref!(ref slice) => slice.iter().sum(),
+        _ => 2000,
+    }
+}
+
+fn nested_vec(vecvec: Vec<Vec<u32>>) -> u32 {
+    match vecvec {
+        deref!([]) => 0,
+        deref!([deref!([x])]) => x,
+        deref!([deref!([0, x]) | deref!([1, x])]) => x,
+        deref!([ref x]) => x.iter().sum(),
+        deref!([deref!([]), deref!([1, x, y])]) => y - x,
+        _ => 2000,
+    }
+}
+
+fn main() {
+    assert_eq!(simple_vec(vec![1]), 1);
+    assert_eq!(simple_vec(vec![1, 2]), 202);
+    assert_eq!(simple_vec(vec![1, 2, 3]), 6);
+
+    assert_eq!(nested_vec(vec![vec![0, 42]]), 42);
+    assert_eq!(nested_vec(vec![vec![1, 42]]), 42);
+    assert_eq!(nested_vec(vec![vec![1, 2, 3]]), 6);
+    assert_eq!(nested_vec(vec![vec![], vec![1, 2, 3]]), 1);
+}

tests/ui/pattern/deref-patterns/branch.rs

@@ -0,0 +1,40 @@
+//@ run-pass
+// Test the execution of deref patterns.
+#![feature(deref_patterns)]
+#![allow(incomplete_features)]
+
+fn branch(vec: Vec<u32>) -> u32 {
+    match vec {
+        deref!([]) => 0,
+        deref!([1, _, 3]) => 1,
+        deref!([2, ..]) => 2,
+        _ => 1000,
+    }
+}
+
+fn nested(vec: Vec<Vec<u32>>) -> u32 {
+    match vec {
+        deref!([deref!([]), ..]) => 1,
+        deref!([deref!([0, ..]), deref!([1, ..])]) => 2,
+        _ => 1000,
+    }
+}
+
+fn main() {
+    assert!(matches!(Vec::<u32>::new(), deref!([])));
+    assert!(matches!(vec![1], deref!([1])));
+    assert!(matches!(&vec![1], deref!([1])));
+    assert!(matches!(vec![&1], deref!([1])));
+    assert!(matches!(vec![vec![1]], deref!([deref!([1])])));
+
+    assert_eq!(branch(vec![]), 0);
+    assert_eq!(branch(vec![1, 2, 3]), 1);
+    assert_eq!(branch(vec![3, 2, 1]), 1000);
+    assert_eq!(branch(vec![2]), 2);
+    assert_eq!(branch(vec![2, 3]), 2);
+    assert_eq!(branch(vec![3, 2]), 1000);
+
+    assert_eq!(nested(vec![vec![], vec![2]]), 1);
+    assert_eq!(nested(vec![vec![0], vec![1]]), 2);
+    assert_eq!(nested(vec![vec![0, 2], vec![1, 2]]), 2);
+}

tests/ui/pattern/deref-patterns/cant_move_out_of_pattern.rs

@@ -0,0 +1,24 @@
+#![feature(deref_patterns)]
+#![allow(incomplete_features)]
+
+use std::rc::Rc;
+
+struct Struct;
+
+fn cant_move_out_box(b: Box<Struct>) -> Struct {
+    match b {
+        //~^ ERROR: cannot move out of a shared reference
+        deref!(x) => x,
+        _ => unreachable!(),
+    }
+}
+
+fn cant_move_out_rc(rc: Rc<Struct>) -> Struct {
+    match rc {
+        //~^ ERROR: cannot move out of a shared reference
+        deref!(x) => x,
+        _ => unreachable!(),
+    }
+}
+
+fn main() {}

tests/ui/pattern/deref-patterns/cant_move_out_of_pattern.stderr

@@ -0,0 +1,27 @@
+error[E0507]: cannot move out of a shared reference
+  --> $DIR/cant_move_out_of_pattern.rs:9:11
+   |
+LL |     match b {
+   |           ^
+LL |
+LL |         deref!(x) => x,
+   |                -
+   |                |
+   |                data moved here
+   |                move occurs because `x` has type `Struct`, which does not implement the `Copy` trait
+
+error[E0507]: cannot move out of a shared reference
+  --> $DIR/cant_move_out_of_pattern.rs:17:11
+   |
+LL |     match rc {
+   |           ^^
+LL |
+LL |         deref!(x) => x,
+   |                -
+   |                |
+   |                data moved here
+   |                move occurs because `x` has type `Struct`, which does not implement the `Copy` trait
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0507`.

tests/ui/pattern/deref-patterns/typeck.rs

@@ -4,6 +4,8 @@
 
 use std::rc::Rc;
 
+struct Struct;
+
 fn main() {
     let vec: Vec<u32> = Vec::new();
     match vec {
@@ -22,10 +24,12 @@ fn main() {
         deref!(1..) => {}
         _ => {}
     }
-    // FIXME(deref_patterns): fails to typecheck because `"foo"` has type &str but deref creates a
-    // place of type `str`.
-    // match "foo".to_string() {
-    //     box "foo" => {}
-    //     _ => {}
-    // }
+    let _: &Struct = match &Rc::new(Struct) {
+        deref!(x) => x,
+        _ => unreachable!(),
+    };
+    let _: &[Struct] = match &Rc::new(vec![Struct]) {
+        deref!(deref!(x)) => x,
+        _ => unreachable!(),
+    };
 }

tests/ui/pattern/deref-patterns/typeck_fail.rs

@@ -0,0 +1,17 @@
+#![feature(deref_patterns)]
+#![allow(incomplete_features)]
+
+fn main() {
+    // FIXME(deref_patterns): fails to typecheck because `"foo"` has type &str but deref creates a
+    // place of type `str`.
+    match "foo".to_string() {
+        deref!("foo") => {}
+        //~^ ERROR: mismatched types
+        _ => {}
+    }
+    match &"foo".to_string() {
+        deref!("foo") => {}
+        //~^ ERROR: mismatched types
+        _ => {}
+    }
+}

tests/ui/pattern/deref-patterns/typeck_fail.stderr

@@ -0,0 +1,19 @@
+error[E0308]: mismatched types
+  --> $DIR/typeck_fail.rs:8:16
+   |
+LL |     match "foo".to_string() {
+   |           ----------------- this expression has type `String`
+LL |         deref!("foo") => {}
+   |                ^^^^^ expected `str`, found `&str`
+
+error[E0308]: mismatched types
+  --> $DIR/typeck_fail.rs:13:16
+   |
+LL |     match &"foo".to_string() {
+   |           ------------------ this expression has type `&String`
+LL |         deref!("foo") => {}
+   |                ^^^^^ expected `str`, found `&str`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.